The fabrication of integrated circuits includes a step of providing a substrate, such as a silicon or gallium arsenide wafer having a smooth uniform surface on one side thereof. Various layers, which are either conductive, insulating or semiconductor in nature are then formed on top of the smooth surface. In fabricating such circuits, it is also necessary to form conductive lines or similar structures above a previously formed layer. However, prior surface formations usually leave the top surface topography highly irregular, with bumps, areas of unequal elevation, troughs and other surface irregularities. As a result, global planarization is necessary to ensure adequate focal depth during subsequent photolithography, as well as removing any irregularities and surface imperfections during the various stages of the fabrication process. This planarization is generally accomplished by chemical mechanical planarization (CMP).
The process of chemical mechanical planarization is carried out using aqueous slurries of suitable chemicals and abrasive particles. The chemicals react with the surfaces being polished to form a reaction layer which is then abraded, or rubbed off, by the solid particles contained in the slurry. In many cases these particles are composed of silica. Heretofore, a type of silica known as fumed silica has been the principal source for these particles. Fumed silica is produced by the flame hydrolysis or burning oxidation of silane compounds such as SiCl4, HSiCl.3, CH3, SiCl3, CH3, Si(OCH3), and the like. The ultimate, spherical, silica particles produced by the flame hydrolysis or burning oxidation of silane compounds are very small, on the order of 10 to 20 nanometers.
These very small particles aggregate while molten and fumed silica is recovered as tightly bound, or fused, chain-like agglomerates of these ultimate particles. The effective particle diameter of these non-spherical agglomerates after dispersion into a CMP slurry is on the order of 100 nm. Both during the preparation of dispersions from these particles, and in their employment for CMP, the chain-like agglomerates are randomly fractured to produce asperities which can plow or dig into the surface being polished, producing undesirable gouges and scratches. Scratches and gouges remaining in the surface of an interlayer of an integrated circuit after planarization are extremely undesirable because they adversely affect the reliability of subsequent layers, acting as sources of defects and sites for contamination.
Also, the plowing or digging action of fumed silica particles results in some of the particles remaining partially embedded in the polished surface at the conclusion of the CMP process. Therefore, cleaning of the polished surface is difficult, often requiring vigorous mechanical brushing during the cleaning process. For example, particle contamination of the polished surface of a memory disc, can produce read/write errors. CMP slurries produced with fumed silica have an additional drawback in that they contain traces of chlorine resulting from the silica particles having been produced from chlorosilanes. Chlorides are an especially undesirable contaminant in integrated circuit manufacture.
In efforts to overcome the scratching, gouging and cleaning problems associated with fumed silica particles, spherical silica particles have been employed in CMP slurries. Such spherical particles have been obtained by the well-known technique of subjecting an aqueous sodium or potassium silicate solution to ion-exchange to produce ultrafine silica particles which are subsequently grown in size by Oswalt ripening, or by the hydrolysis of ethyl silicate using the so-called Stober process. The Stober process produces particles having a three-dimensional condensation structure. The Stober process is disclosed in an article “Controlled Growth of Monodispersed Silica Spheres in a Micron Range,” by Stober et al published in the Journal of Colloid and Interface Sci. 26, 62-69 (1968).
While the use of such spherical particles reduces the scratching and cleaning problems associated with slurries made with fumed silica particles, the material removal rate of such slurries, for equal particle concentrations, is dramatically lower. This is associated, of course, with the fact that the removal rate of the chemically reacted layer is a function of the frictional force between it and the abrasive particle. While this reduced removal rate can be mitigated to a degree by significantly increasing the particle concentration of the spherical particles in the CMP slurry, this is at the expense of additional material cost, cleaning cost and spent-slurry disposal cost. Also, the slurries made from sodium or potassium silicate have these alkali metals as a contaminant, which are particularly undesirable in the manufacture of integrated circuits.
A further approach to solve the aforementioned problems is described in a U.S. Pat. No. 6,334,880 of Negrych et al. As disclosed therein, non-spherical particles having nodular morphology are used as an abrasive media in chemical mechanical polishing. Also disclosed are aqueous slurries of mono-dispersed non-spherical nodular shaped particles having mean effective diameters between about 100 and 300 nanometers for chemical mechanical polishing and aqueous slurries for the chemical mechanical polishing and planarization of oxide, dialectric, metal and metal/metallic compound interlayers of integrated circuits.
A more recent approach to silicate particles for polishing a semiconductor integrated circuit is disclosed in a U.S. Pat. No. 6,652,612 of Nakayama et al. As disclosed therein, silica particles for polishing have a three-dimensional poly condensation structure with an average particle diameter in a range of 5 to 300 nm. The silica particles have residual alkoxy groups therein and a carbon content in a range from 0.5 to 5 weight percent retained in the residual alkoxy groups.
Notwithstanding the above, it is presently believed that there is a commercial market for a stabilized suspension of nanopowders in accordance with the present invention. It is believed that there will be a commercial market for such products because they provide a smoother polished surface and a relatively good polishing rate without scratches and a more uniform particle size of between about 5 nm and 20 nm. Further it is believed that the rate of polishing is further enhanced by a mix of spherical and near spherical particles.